This invention relates to flexible polyurethane foams. More particularly it relates to flexible polyurethane foams having low resistance to air flow while having excellent load bearing characteristics and high strength, rendering the foam particularly useful as an air filter foam.
Polyurethane foam is used widely as a flexible cellular product in the comfort cushion market (furniture, bedding, automotive); in the textile area (apparel, blankets); in the industrial packaging and insulating fields; in other household furnishings and sponges; filters, and the like. The versatility of polyurethane foam, permitting its use in diverse markets, results in substantial part from the nature and variety of the raw materials which are used to produce the foam products, as well as the manner in which the raw materials and the resultant foam are processed. Foams ranging widely in density and hardness, in tensile and tear properties, in resistance to compression set and fatigue, in resilience and hysteresis, in durability and toughness are obtained by selection and variation in raw materials and processing conditions. An important further characteristic in foam that likewise varies widely is its breathability, or resistance to air flow, of the basic cellular structure. In the past efforts have been directed to the production of polyurethane foams, for example, for gasket applications, in which the foam was required to have high resistance to air flow. Similarly, foams have been produced with a reticulated structure, i.e., a structure wherein all the cell membranes are removed, so that the foam will exhibit very low resistance to air flow and permit the foams to be used commercially as an air filter. Flexible polyurethane foams having a reticulated structure are described in U.S. Pat. Nos. 3,171,820; 3,175,025; 3,884,848 and 3,862,282. The reticulated structures are provided by post treatment of the foams. While the reticulated foams heretofore produced have utility in various applications, the nature of the post-treatment methods used to produce such foams result in certain limitations. For example, the post-treatment methods add significantly to the cost of the foam. Moreover, the foam is weakened or softened as compared to the non-reticulated precursor due to the removal of the cell membranes.
It is also known to produce foams having improved breathability by eliminating or reducing the use of certain gellation catalysts such as the tin catalysts exemplified by stannous octoate. The foams are produced from conventional formulations other than for the catalyst modification. However, their processing must be rigorously controlled. Moreover, although the foams obtained have more open-celled structures, it is not possible to consistently produce large blocks of foam having uniformly high breathability throughout the dimensions of the block. Furthermore, the breathability of the foam must be restricted, otherwise large voids are formed during the blowing of the block, resulting in major amounts of scrap material. Additionally, such foams are prone to become unduly soft and exhibit abnormally high compression sets.